Multi-layer work function metal replacement gate

ABSTRACT

Embodiments relate to a field-effect transistor (FET) replacement gate apparatus. The apparatus includes a channel structure including a base and side walls defining a trench. A high-dielectric constant (high-k) layer is formed on the base and side walls of the trench. The high-k layer has an upper surface conforming to a shape of the trench. A first layer is formed on the high-k layer and conforms to the shape of the trench. The first layer includes an aluminum-free metal nitride. A second layer is formed on the first layer and conforms to the shape of the trench. The second layer includes aluminum and at least one other metal. A third layer is formed on the second layer and conforms to the shape of the trench. The third layer includes aluminum-free metal nitride.

BACKGROUND

The present disclosure relates to a multi-layer work function metalreplacement gate, and in particular to layers of work function metalsthat conform to a shape of a trench structure and which are variable toadjust work function levels of a replacement gate structure.

Field-effect transistors (FETs) generate an electric field, by a gatestructure, to control the conductivity of a channel between source anddrain structures in a semiconductor substrate. The source and drainstructures may be formed by doping the semiconductor substrate, and thegate may be formed on the semiconductor substrate between the source anddrain regions. Alternatively, a source and drain structure may be formedon the substrate, and a channel may extend between the source and thedrain on the semiconductor substrate. In such a structure, referred toas a finFET due to the fin-like shape of the channel, the gate structuremay be formed on the channel.

The gate of a finFET, and in some non-finFETs, may be formed by areplacement gate process, or a process in which material, such as dummymaterial, is removed to form a trench, and the gate materials replacethe removed material in the trench. In a finFET, the trench may bedefined by a plurality of channels and the source and drain structures.In other FETs, as well as in finFETs, the trench may be formed byinsulating separators, for example. The gate may be formed by depositinga work function metal in the trench and forming a metal gap fill on thework function metal. Titanium aluminum (TiAl) has been used as areplacement gate work function metal, but TiAl has been limited tonon-conformal methods of application, such as physical vapor deposition(PVD), in which an upper surface of the deposited material does notconform to a shape of the surface on which the material is deposited,making TiAl less-than-ideal as a replacement gate work function metal.In addition, use of Al-based metal electrodes causes gate leakagecurrent degradation due to a strong oxygen gettering effect.

SUMMARY

Exemplary embodiments include a field-effect transistor (FET)replacement gate apparatus. The apparatus includes a channel structureincluding a base and side walls defining a trench. A high-dielectricconstant (high-k) layer is formed on the base and side walls of thetrench. The high-k layer has an upper surface conforming to a shape ofthe trench. A first layer is formed on the high-k layer. The first layerconforms to the shape of the trench. The first layer includes analuminum-free metal nitride. A second layer is formed on the first layerand conforms to the shape of the trench. The second layer includesaluminum and at least one other metal. A third layer is formed on thesecond layer and conforms to the shape of the trench. The third layerincludes an aluminum-free metal nitride.

Additional exemplary embodiments include a field-effect transistorreplacement gate apparatus. The apparatus includes a substrate and sidewalls extending from the substrate to form a trench. A high dielectricconstant (high-k) layer is formed on at least the substrate. A firstlayer is formed on the high-k layer. The first layer includes analuminum-free metal nitride. A second layer is formed on the firstlayer. The second layer includes aluminum and at least one other metal.The ratio of the aluminum to the at least one other metal is a gradientwith a peak located in a center region of the second layer and troughslocated at ends of the second layer. The third layer is formed on thesecond layer. The third layer includes an aluminum-free metal nitride.

Additional features and advantages are realized through the techniquesof the present disclosure. Other embodiments and aspects of the presentdisclosure are described in detail herein and are considered a part ofthe claimed disclosure. For a better understanding of the disclosurewith the advantages and the features, refer to the description and tothe drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter of the disclosure is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The forgoing and other features, and advantages of the disclosure areapparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 illustrates a replacement gate structure according to oneembodiment of the present disclosure;

FIG. 2A illustrates a ratio of aluminum to another metal in a layer of agroup of work function metals according to one embodiment;

FIG. 2B illustrates a ratio of aluminum to another metal in a layer of agroup of work function metals according to another embodiment;

FIGS. 3A-3E illustrate a method of forming the replacement gatestructure according to one embodiment, in which:

FIG. 3A illustrates forming a high-dielectric-constant material on asubstrate;

FIG. 3B illustrates forming a first layer of a group of work functionmetal layers;

FIG. 3C illustrates forming a second layer of a group of work functionmetal layers;

FIG. 3D illustrates forming a third layer of a group of work functionmetal layers;

FIG. 3E illustrates forming a gap fill metal layer; and

FIG. 4 illustrates a flowchart of a method for forming the replacementgate structure according to one embodiment.

DETAILED DESCRIPTION

Conventional replacement gate work function metals suffer from gateleakage current degradation due to material types and processes forapplying the materials. Disclosed embodiments relate to work functionmetal layers that conform to a shape of a replacement gate trench,reduce gate leakage current, and may have an adjustable work functionvalue.

FIG. 1 illustrates a replacement gate field-effect transistor (FET)structure 100 according to one embodiment of the present disclosure. Thestructure 100 includes a substrate 101, which may be a semiconductorsubstrate, such as a silicon substrate for example. The structure 100includes side walls 102 extending from the substrate 101. In oneembodiment, the substrate is a doped semiconductor substrate 101 havingbeen doped to include source and drain regions (not shown). In such anembodiment, the side walls 102 may be insulators. In another embodiment,the structure 100 is a finFET structure, and the side walls 102 comprisechannels or fins 102 extending between a source and a drain structureformed on the substrate 101. Alternatively, the side walls 102 may bethe source and drain structures formed on the substrate 101. Thesubstrate 101 and side walls 102 define a trench 103.

The structure 100 further includes a high-dielectric constant (high-k)layer 104 formed on the side walls 102 and on the substrate 101. Thehigh-k layer 104 may be formed directly on the substrate, for example.In one embodiment, the high-k layer 104 includes hafnium (Hf), such ashafnium dioxide (HfO₂). In one embodiment, the high-k layer 104 isformed to conform to the shape of the trench 103. For example, thehigh-k layer 104 may be formed by an atomic layer deposition (ALD)process which results in a conforming layer.

The structure illustrated in FIG. 1 may correspond, for example, to afinFET in which the side walls 102 are channels extending between asource structure and a drain structure, or the side walls 102 may be thesource structure and the drain structure. However, embodiments of thepresent disclosure also encompass planar FET embodiments in which theside walls 102 are insulation layers. In such a case, the high-k layer104 may be formed either to conform to the side walls 102 or may beformed only at the base of the trench 104.

The structure 100 further includes a work function metal layer group105. The work function metal layer group 105 includes a first layer 106formed on the high-k layer 104, a second layer 107 formed on the firstlayer 106, and a third layer 108 formed on the second layer 107. In oneembodiment, the first layer 106 is formed directly on the high-k layer104, the second layer 107 is formed directly on the first layer 106, andthe third layer 108 is formed directly on the second layer 107. In oneembodiment, the first and third layers comprise an aluminum-free metalnitride layer. For example, the first and third layers 106 and 108 maybe titanium nitride (TiN) or tantalum nitride (TaN). In one embodiment,the first layer 106 does not include oxygen. In one embodiment, thesecond layer 107 is a metal layer including aluminum and at least oneother metal. For example, the second layer 107 may be made up oftitanium and aluminum (TiAl) without nitrogen, or the second layer maybe made up of titanium, aluminum and nitrogen (TiAlN).

The second layer 107 may be formed to have varying ratios of aluminum(Al) to another metal. Titanium (Ti) will be described in the followingexample for purposes of clarity. However, embodiments of the presentdisclosure encompass any appropriate metal in combination with aluminum.The ratio of Al:(Al+Ti) may be adjusted to adjust a work function of thestructure 100. In one embodiment, a ratio of Al:(Al+Ti) is substantiallyconstant throughout the entire second layer 107. The second layer 107may be formed by ALD, and the ratio may be maintained constant bydepositing layers of Al and Ti in a particular sequence. In oneembodiment, the ratio of Al:(Al+Ti) in the second layer is a gradienthaving a peak at a center portion of the layer. The center portion maycorrespond, for example, to about +/−10% of the height of the secondlayer 107 from a center plane of the second layer 107. In such anembodiment, layers of Al may be deposited in an ALD process with agreater frequency when forming the center portion of the second layer107 than when forming the end portions.

FIGS. 2A and 2B illustrate the ratio of Al to Al+Ti according toembodiments of the present disclosure. As illustrated in FIG. 2A, in oneembodiment a ratio of Al to Al+Ti is zero in regions corresponding tothe first and third layers 106 and 108, since these layers include noAl. In the region corresponding to the second layer 107, the ratio of Alto Al+Ti is constant. In other words, when forming the second layer 107by ALD, a sequence of deposition of Al and Ti layers may be maintainedconstant throughout the formation of the second layer 107.

As illustrated in FIG. 2B, in another embodiment, a ratio of Al to Al+Tiis still zero in regions corresponding to the first and third layers 106and 108, since these layers include no Al. However, in the regioncorresponding to the second layer 107, the ratio of Al to Al+Ti is agradient that increases from the edges of the second layer 107 andreaches a peak at a center region of the second layer 107. In otherwords, when forming the second layer 107 by ALD, a sequence ofdeposition of Al and Ti layers may be maintained altered so that layersof Al are deposited with increased frequency relative to layers of Ti inthe center region of the second layer 107.

Referring again to FIG. 1, in addition to controlling the work functionof the structure 100 based on the ratio of Al:(Al+Ti) in the secondlayer 107, embodiments of the present disclosure further encompasscontrolling the work function of the structure 100 based on a thicknessof the first layer 106. In one embodiment, the thickness of the firstlayer 106 is formed or designed such that the work function of the workfunction metal layers 105 corresponds to a quarter-gap work function.Embodiments of the present disclosure further encompass controlling gateleakage current levels by controlling the thickness of the first layer106 and the ratio of Al:(Al+Ti) in the second layer 107.

In one embodiment, the first layer 106 has a thickness between about 10angstroms (Å) and about 30 Å, the second layer 107 has a thicknessbetween about 10 A and about 60 A, and the third layer 108 has athickness between about 10 Å and 30 Å.

The structure 100 further includes a gap fill metal 109 formed on thethird layer 108. In one embodiment, the gap fill metal 109 is formeddirectly on the third layer 108. The gap fill metal 109 may be anon-conforming metal, or may be formed by a non-conforming process, suchas PVD. Alternatively, the gap fill metal 109 may also be formed by aconforming process, such as ALD or chemical vapor deposition (CVD). Inone embodiment, the gap fill metal 109 is aluminum. However, embodimentsof the present disclosure encompass any conductive metal.

FIGS. 3A to 3E illustrate a process for forming a replacement gatestructure 100 according to an embodiment of the disclosure. FIGS. 3A to3E illustrate a portion of the replacement gate structure 100 around onereplacement gate structure 100. However, it is understood that thedescribed layers may be of any length and width dimensions, and multiplereplacement gate structures 100 may be formed. FIG. 4 is a flow diagramof a method of forming a replacement gate structure according to anembodiment of the present disclosure. The formation of the structure 100will be described below with reference to FIGS. 3A to 3E and 4.

In block 401 of FIG. 4, a substrate 100 is formed and side walls 102 areformed. The substrate 101 may be a semiconductor substrate or a siliconsubstrate. The substrate may be a doped semiconductor substrate 101having been doped to include source and drain regions (not shown). Insuch an embodiment, the side walls 102 may be insulators. In anotherembodiment, the structure 100 is a finFET structure, and the side walls102 comprise channels or fins 102 extending between a source and a drainstructure formed on the substrate 101. Alternatively, the side walls 102may be the source and drain structures formed on the substrate 101. Thesubstrate 101 and side walls 102 define a trench 103.

In block 402 and in FIG. 3A, a high-dielectric constant (high-k) layer104 is formed on the substrate 101 and side walls 102. The high-k layer104 may be formed directly on the substrate 101, for example. In oneembodiment, the high-k layer 104 includes hafnium (Hf), such as hafniumdioxide (HfO₂). In one embodiment, the high-k layer 104 is formed toconform to the shape of the trench 103. For example, the high-k layer104 may be formed by an atomic layer deposition (ALD) process whichresults in a conforming layer. The ALD process is represented by arrowsin FIGS. 3A to 3D.

In block 403 and in FIG. 3B, a first layer 106 is formed on the high-klayer 104. The first layer 106 may be formed directly on the high-klayer 104. The first layer 106 may be formed by a conforming process. Inone embodiment, the first layer 106 is formed by ALD. The first layermay be an aluminum-free metal nitride layer. For example, the firstlayer 106 may be titanium nitride (TiN) or tantalum nitride (TaN). Inone embodiment, the first layer 106 does not include oxygen and is notmodified during fabrication of the structure 100 to include oxygen. Inone embodiment, a height of the first layer 106 is adjusted to adjust awork function of the work function metal group 105 (see FIGS. 1 and 3E).In one embodiment, the thickness of the first layer 106 is formed suchthat the work function of the work function metal group 105 correspondsto a quarter-gap work function. In one embodiment, the first layer 106has a thickness between about 10 Å and about 30 Å.

In block 404 and in FIG. 3C, a second layer 107 is formed on the firstlayer 106. The second layer 107 may be formed directly on the firstlayer 106. The second layer 107 may be formed by a conforming process.In one embodiment, the second layer 107 is formed by ALD. In oneembodiment, the second layer 107 is a metal layer including aluminum andat least one other metal. For example, the second layer 107 may be madeup of titanium and aluminum (TiAl) without nitrogen. Alternatively, thesecond layer may be made up of titanium, aluminum and nitrogen (TiAlN).The second layer 107 may be formed by applying layers of Al and one ormore additional metals in sequential atomic layers in an ALD process. Inan embodiment in which the second layer 107 comprises TiAl, layers of Tiand Al may be deposited in sequence in predetermined ratios.

In the embodiment in which the second layer includes TiAl, the ratio ofAl:(Al+Ti) may be adjusted to adjust a work function of the structure100. In one embodiment, a ratio of Al:(Al+Ti) is substantially constantthroughout the entire second layer 107. In other words, layers of Al andTi are deposited by an ALD process in constant ratios. In oneembodiment, the ratio of Al:(Al+Ti) in the second layer is a gradienthaving a peak at a center portion of the layer. The center portion maycorrespond, for example, to about +/−10% of the height of the secondlayer 107 from a center plane of the second layer 107. In such anembodiment, layers of Al may be deposited in an ALD process with agreater frequency when forming the center portion of the second layer107 than when forming the end portions, relative to a frequency withwhich the Ti layers are deposited.

In one embodiment, the percentage of Al relative to Al+Ti in the secondlayer 107 is between about 10% and about 90%. In one embodiment, thesecond layer 107 is formed by depositing layers of titanium nitride(TiN) and titanium aluminum nitride (TiAlN) in a particular sequence toobtain a layer of TiAlN having a predetermined ratio of Al:Ti, or apredetermined gradient of ratios of Al:Ti throughout the second layer107. The second layer 107 may be formed to have a thickness between 10 Åand 60 Å.

In block 405 and in FIG. 3D, a third layer 108 is formed on the secondlayer 107. The third layer 108 may be formed directly on the secondlayer 107. The third layer 108 may be formed by a conforming process. Inone embodiment, the third layer 108 is formed by ALD. The third layermay be an aluminum-free metal nitride layer. For example, the thirdlayer 108 may be titanium nitride (TiN) or tantalum nitride (TaN). Inone embodiment, the third layer 108 has a thickness between about 10 Aand about 30 A. The formation of the third layer 108 may preventundesired oxidation of the second layer 107 by air exposure.

In one embodiment, the first, second and third layers 106, 107 and 108are formed in situ, or in a same chamber in sequential order, withoutexposing the chamber to external air between the deposition processes ofthe respective layers. In other words, since the first, second and thirdlayers 106, 107 and 108 may all be formed by ALD, they may all beperformed in the same chamber without exposing the layers to air, andundesired oxidation of the layers 106, 107 and 108 may be avoided.

In block 406 and in FIG. 3E a gap fill metal 109 is formed on the thirdlayer 108. The gap fill metal 109 may be formed directly on the thirdlayer 108. The gap fill metal 109 may be any conductive metal, such asaluminum or tungsten. The gap fill metal 109 may be formed in aconforming process, such as ALD, or a non-conforming process, such asPVD. In addition, a final replacement gate structure 100 may be formedby removing, or polishing off, the top surface layers down to the topsurface of the side walls 102 by chemical mechanical polish, forexample. The final replacement gate structure 100 is illustrated in FIG.1.

Embodiments of the present disclosure encompass a multi-layered workfunction metal group of a replacement gate structure. The work functionmetal group includes a layer of aluminum and at least one other metalbetween two layers of a metal nitride that does not contain aluminum.The layers are formed on a high-k layer, and all of the layers areformed by an ALD process to conform to a shape of a substrate and sidewalls on which the layers are formed. The layer including aluminum andat least one other metal may have a higher concentration of aluminumtowards a center of the layer relative to the edges of the layer. Theconcentration of aluminum may be adjusted according to predetermineddesigns to achieve a particular work function, and to reduce gateleakage current. In addition, the top-most aluminum-free metal nitridelayer prevents undesired oxidation of the aluminum-containing layer byair exposure. In addition, the entire metal group, and the high-k layer,may be formed by ALD to be compatible with replacement gates, such asfinFET structures.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof embodiments of the present disclosure. It is understood that in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present disclosure has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the disclosure in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the disclosure. Theembodiment was chosen and described in order to best explain theprinciples of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various embodiments with various modifications as are suited to theparticular use contemplated.

While exemplary embodiments of the disclosure have been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the disclosure first described.

1. A field-effect transistor (FET) replacement gate apparatus,comprising: a channel structure including a base and side walls defininga trench; a high-dielectric constant (high-k) layer formed on the baseand side walls of the trench and having an upper surface conforming to ashape of the trench; a first layer formed on the high-k layer andconforming to the shape of the trench, the first layer comprising analuminum-free metal nitride; a second layer formed on the first layerand conforming to the shape of the trench, the second layer comprisingaluminum and at least one other metal, a ratio of aluminum to the atleast one other metal is a gradient having a higher ratio at a centerportion of the second layer and a lower ratio at ends of the secondlayer; and a third layer formed on the second layer and conforming tothe shape of the trench, the third layer comprising an aluminum-freemetal nitride.
 2. The apparatus of claim 1, wherein the channelstructure comprises at least one of a substrate and an insulator formedon the substrate.
 3. The apparatus of claim 1, wherein the first andthird layers comprise one of titanium nitride (TiN) and tantalum nitride(TaN).
 4. The apparatus of claim 1, wherein the second layer includestitanium and aluminum (TiAl).
 5. The apparatus of claim 1, wherein thesecond layer does not include nitrogen.
 6. (canceled)
 7. (canceled) 8.The apparatus of claim 1, wherein the aluminum and the at least oneother metal comprise atomic layers of the aluminum and the at least oneother metal, and the atomic layers of the aluminum exist in greaterconcentrations in the center portion of the second layer relative to theends of the second layer.
 9. The apparatus of claim 1, furthercomprising a metal gap fill layer formed on the third layer to fill agap defined by the trench via the high-k layer, and the first, second,and third layers.
 10. The apparatus of claim 1, wherein the FET is afinFET and the side walls are at least one of source and drainstructures and channels extending between the source and drainstructures.
 11. The apparatus of claim 1, wherein the first layerincludes no oxygen.
 12. A field-effect transistor replacement gateapparatus, comprising: a substrate and side walls extending from thesubstrate to form a trench; a high dielectric constant (high-k) layerformed on at least the substrate; a first layer formed on the high-klayer, the first layer comprising an aluminum-free metal nitride; asecond layer formed on the first layer, the second layer comprisingaluminum and at least one other metal, a ratio of the aluminum to the atleast one other metal being a gradient with a peak located in a centerregion of the second layer and troughs located at ends of the secondlayer; and a third layer formed on the second layer, the third layercomprising an aluminum-free metal nitride.
 13. The apparatus of claim12, wherein the high-k layer includes hafnium.
 14. The apparatus ofclaim 12, wherein the first and third layers comprise one of titaniumnitride (TiN) and tantalum nitride (TaN).
 15. The apparatus of claim 12,wherein the second layer includes titanium and aluminum (TiAl).
 16. Theapparatus of claim 12, wherein the second layer does not includenitrogen.
 17. The apparatus of claim 12, wherein layers of aluminumoccur with an increased frequency in the center region of the secondlayer relative to the ends of the second layer.
 18. The apparatus ofclaim 12, further comprising a metal gap fill layer formed on the thirdlayer to fill a gap defined by the trench via the high-k layer, and thefirst, second, and third layers.
 19. The apparatus of claim 12, whereinthe FET is a finFET and the side walls are at least one of source anddrain structures and channels extending between the source and drainstructures.
 20. The apparatus of claim 12, wherein the first layerincludes no oxygen.